fix(backend): allow /tmp as valid path in E2B sandbox file tools (#12501)

## Summary
- Allow `/tmp` as a valid writable directory in E2B sandbox file tools
(`write_file`, `read_file`, `edit_file`, `glob`, `grep`)
- The E2B sandbox is already fully isolated, so restricting writes to
only `/home/user` was unnecessarily limiting — scripts and tools
commonly use `/tmp` for temporary files
- Extract `is_within_allowed_dirs()` helper in `context.py` to
centralize the allowed-directory check for both path resolution and
symlink escape detection

## Changes
- `context.py`: Add `E2B_ALLOWED_DIRS` tuple and `E2B_ALLOWED_DIRS_STR`,
introduce `is_within_allowed_dirs()`, update `resolve_sandbox_path()` to
use it
- `e2b_file_tools.py`: Update `_check_sandbox_symlink_escape()` to use
`is_within_allowed_dirs()`, update tool descriptions
- Tests: Add coverage for `/tmp` paths in both `context_test.py` and
`e2b_file_tools_test.py`

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] All 59 existing + new tests pass (`poetry run pytest
backend/copilot/context_test.py
backend/copilot/sdk/e2b_file_tools_test.py`)
  - [x] `poetry run format` and `poetry run lint` pass clean
  - [x] Verify `/tmp` write works in live E2B sandbox
  - [x] E2E: Write file to /tmp/test.py in E2B sandbox via copilot
  - [x] E2E: Execute script from /tmp — output "Hello, World!"
  - [x] E2E: E2B sandbox lifecycle (create, use, pause) works correctly